Studies are focused on an understanding of the molecular biology and molecular genetics of the retina and hereditary retinal degenerations. The retina and pigment epithelium are neuroepithelial tissues that work in close cooperation. Specific growth and differentiating factors found in the eye guide development and interactions of individual ocular tissues to form a functional visual system. For example, ocular tissues synthesize a number of growth factors. There now appear to be several systems that could self-regulate growth and metabolic activity in the retinal pigment epithelium and that could be involved in eye diseases. In this regard, we have cloned and characterized a unique differentiating protein secreted from fetal human pigment epithelial cells, called pigment-epithelium-derived factor, that is neurotrophic to cultured human retinoblastoma cells and may affect neural retinal development in vivo. This protein maps to chromosome 17p, where there is a cluster of cancer- related genes. It is a prime candidate in the hereditary retinal dystrophy observed in the Royal College of Surgeons rat.